1. Field of the Invention
The present invention relates to a correction technique of recording power during data recording into an optical disk.
2. Description of the Related Art
An optical disk such as CD-R (write-once type CD), DVD±R (write-once type DVD), HD-DVD-R (write-once type HD-DVD), or BD-R (write-once type Blu-ray disc) has a structure where a recording layer, reflective layer, and protective layer as needed are formed on one surface of an optical transparency disk substrate. Moreover, spiral or concentric-circle grooves are formed on the surface on which the recording layer and the reflective layer are formed, and a portion between adjacent grooves is formed to be a convex portion called land. In such an optical disk, a recording laser beam forms pits in the recording layer by irradiating the recording layer over the grooves with the beam tracking along the grooves by a recording and reproduction device for optical disk. While the pits have a length nT (bit length between respective reference channel clocks is assumed to be T, and length of integral multiples of n of the bit length is assumed to be nT), and portions between the pits (hereinafter, called spaces) also have a length nT, arrays of them are irradiated with a laser beam for reproduction, and a reflected beam is converted into a reproduction signal, so that reproduction is performed.
The recording and reproduction device for optical disk is designed so as to meet a recording condition that is varied every time when recording is performed due to a drive, an optical disk (sometimes called media), recording velocity and the like. To meet the various recording conditions, the recording and reproduction device for optical disk employs a technique for optimally setting laser radiation intensity (hereinafter, called recording power). A device is given as such a technique, which uses OPC (Optimal Power Calibration) as a selection method. In the OPC, test recording is performed into a test area (Power Calibration Area) of a recording disk while changing output of a recording laser beam before data recording is performed. Then, each of results of the test recording is compared to a result based on a previously registered, initial condition, and therethrough best recording power providing excellent recording quality is selected from the results, and set. Recording is performed into a data recording area of the optical disk using the set recording laser beam with the best recording power.
Then, as a parameter showing a recording condition, a value of β, which is one type of asymmetry being an evaluation index showing asymmetry of a waveform formed by reproducing a recording waveform, is calculated from change in recording and reproduction signal when a recording power condition is changed, and the value of β is determined so as to be equal or close to a target value, so that best recording power is obtained for best recording correction.
Moreover, a device employing a technique (ROPC: Running Optimal Power Calibration) is known, in which, to cope with change in characteristic (sensitivity) depending on thickness variation from inner circumference to outer circumference of an optical disk or influence of a warp of the optical disk, a return beam (WRF) to a spot of a recording laser beam is detected during data recording, or a sub-spot provided near a main spot is detected by using optical diffraction during the data recording, thereby the value of β as above, jitter (fluctuation in a time axis direction of a digital signal), or a value of an evaluation index in a correlation with the jitter, is acquired so that the recording power condition is optimized in real time for the optical disk itself or the recording and reproduction device for optical disk.
Furthermore, a device is disclosed as a simple approach of the above technique, which employs a technique (WOPC: Walking Optical Power Calibration) in which during data recording from inner circumference to outer circumference of an optical disk, recording operation is temporarily stopped at a predetermined position on the optical disk, and data area, to which recording is performed immediately before such operation stop, is subjected to reproduction, thereby the value of β, jitter, or the value of the evaluation index in a correlation with the jitter, is acquired in order to optimize the recording power condition. For example, JP-A-2004-234812 is given as a reference.
However, when using WOPC with a DVD (Digital Versatile Disc), an evaluation technique using a value of β as an index does not sufficiently cope with a recording and reproduction system for optical disk for high density recording and reproduction using the PRML (Partial Response Maximum Likelihood) signal processing method (system according to the Blu-ray standard or the HD-DVD standard) and consequently the WOPC cannot appropriately correct recording power in the system.
Moreover, the usual techniques have a problem that they cannot cope with an optical disk for high density recording and reproduction, in which the value of β, or asymmetry as an evaluation index being similar to β but calculated by a different method, is not in a correlation with recording power and consequently the techniques cannot appropriately correct recording power.